1. Field of the Invention
The present invention relates to microelectronic packaging, and more particularly, a fluxing-encapsulant material and use thereof in a thermal compression bonding process.
2. Discussion of Related Art
The ongoing trend toward increased performance and higher density electrical circuits has led to the development of surface mount technology in the design of electronic packages and printed circuit boards (PCBs). Thermal compression bonding (TCB) has gained wide acceptance within the electronic packaging industry to bond a top substrate directly onto a bottom substrate with solder bumps that are reflowed under elevated temperature and pressure to form joints. For example, thermal compression bonding may be used for bonding die-to-die, die-to-substrate carrier, die-to-package and die-to-wafer.
Several implementations of thermal compression bonding are utilized in industry. In a first implementation a fluxing material is applied prior to or during thermal compression bonding to reduce oxides on the joints. An encapsulant is then dispensed after thermal compression bonding to provide mechanical protection for the joints. In the first implementation a deflux process may be required prior to dispensing the encapsulant to wash off flux residues. The joints are unprotected during the deflux process. Voiding and reliability failures may result due to inefficient defluxing associated with tight bump pitches as well as the joints being unprotected during the deflux process.
A second implementation limits the surface finish of the bond pads to materials which do not readily oxidize. In this implementation the use of a fluxing material may be eliminated and the joints are formed without a flux material. An encapsulant may be dispensed during or after thermal compression bonding to provide mechanical protection for the joints. Poor joint quality and voiding may result due to contamination on the bond pads and partial non-wets associated with not fluxing solder oxide.
In a third implementation a no-flow underfill process is used for thermal compression bonding in which an underfill possessing self-fluxing capability is cured during the solder reflow process. However, current no-flow underfill processes are plagued with several failure modes. For example, non-wetting of bond pad and solder metallurgies including copper, nickel, etc. is commonly experienced due to insufficient fluxing action of the no-flow underfill type material. Significant voiding is also observed due to the incompatability of the no-flow underfill type material and cure kinetics of the no-flow underfill type material with fast ramp TCB thermal profiles. This voiding can also lead to joint cracks and insufficient thermomechanical protection of the cured underfill.